Embodiments of the invention generally to electron emissive materials and in particular to barium-free electron emissive materials for electric plasma discharge devices.
Low-pressure metal halide electric discharge plasmas have the potential to replace the mercury-based electric discharge plasma used in conventional fluorescent lamps. However, many known electron emission materials in conventional lamps are not chemically stable in the presence of metal halide plasma. Electron-emissive mixtures containing barium oxide have been typically used in mercury discharge lamps. However, the use of barium oxide in metal halide discharge lamps poses certain challenges. The use of barium oxide as a component of lamp electrodes, especially in low-pressure metal halide discharge lamps, is expected to lead to performance issues. This is at least in part due to the reaction of the metal halide with barium oxide, which can lead to the formation of barium halide and a condensed metal oxide. For example, a metal halide discharge material such as indium bromide may react with an electrode material such as barium oxide to form barium bromide and indium oxide. Such a reaction would lead to a direct reduction in light emitting discharge material present in the discharge plasma. It is therefore advantageous to avoid such deleterious reactions in discharge lamps involving the metal halide emission material, as it may lead to a reduction in life of the lamp.
In conventional fluorescent mercury lamps, due to reactivity problems with components of the electron emissive material such as barium, some amount of mercury may be effectively removed from the discharge medium and hence cannot not contribute to radiation emission. For example, barium in a barium-strontium-calcium triple oxide electron emissive material may amalgamate with mercury in the discharge medium leading to a reduction in the amount of mercury available for radiation emission. To compensate for such loss, higher dosages of mercury, sometimes up to 10 to 50 times higher mercury dosage than the 0.1 mg of mercury typically required, is used to ensure adequate availability of mercury through the life of the lamp.